Location of buried or submerged objects is often tricky for persons situated at the surface. Such location turns out to be necessary in particular with a view to carrying out works, either to access water pipes or gas pipes, or to avoid harming them, or to update network plans. Location based solely on plans made when positioning the pipes usually turns out to be unusable on account of a lack of reliability of the plans, on account of loss of these plans, or on account of an uncontrolled displacement of the pipes (for example subsequent to terrain movements or earthworks).
On account of the absence of direct access to such pipes, a certain number of procedures have been fine-tuned to facilitate their effective location from the surface. A known location procedure consists in particular in fixing RFID tags to pipes at strategic sites beforehand, and then in subsequently locating these tags from the surface by using a reader associated with these tags.
By determining the horizontal position and the depth of the various tags, it is in particular possible to reconstruct in three dimensions an underground network of pipes. The presence of tags at singular points of the network (for example at the level of the pipe branchings or bends) facilitates the reconstruction of its plan.
Several modes of communication are known between a reader and an RFID tag. In such systems, a link is established by radiofrequency magnetic field between the reader and one or more tags.
High frequency and ultra high frequency radiating antenna type antennas, whose size is of the order of half the wavelength of the communication frequency, are sensitive both to the magnetic field component and to the electric field component. Communication between the reader and the tag is very dependent on the structure of the antennas of the tag and of the reader. Moreover, having regard to the distances involved of greater than the wavelength of the electromagnetic field and of the electromagnetic characteristics of the medium in which the tag is buried, the ultra high frequencies lead to electromagnetic field pattern diagrams that are more complex and more liable to be disturbed. The moisture of the medium furthermore induces increased absorption of the waves. Reliability of location is thus seriously affected.
On account of these limitations, the use of inductive type antennas is favored for such applications. The communication between a reader and RFID tags is for example defined in the standards ISO15693 and ISO18000-3 for the frequency of 13.56 MHz.
In the case of inductive antennas, the interactions between the antenna of the reader and the antenna of the tag can be described by equations of inductive coupling (with a quasi-static approach and the use of the calculation of mutual inductances). The inductive coupling induces the transfer of energy between the reader and the tag by mutual inductance.
Across its surface, a coiled conducting circuit of the tag taps off the flux of the magnetic field produced by the antenna of the reader. The temporal variation of this flux creates an induced voltage termed the e.m.f (for electromotive force) within this coiled circuit. This voltage is rectified and generally used to power the functions of the tag.
The coiled circuit of the antenna exhibits an inductance. This inductance is exploited by associating it with a capacitive element added to form a parallel resonator. The voltage available across the terminals of this resonator is then the product of the voltage induced by the overvoltage coefficient (corresponding overall to the coefficient of quality of the resonator) thus allowing the energizing of an integrated circuit of the tag. The remote energizing of an integrated circuit of an RFID identification tag requires a minimum voltage (and power) to operate, typically of the order of a few volts peak and a few hundred microwatts. There thus exists a minimum magnetic field value applied to the antenna of the tag upward of which the tag is functional and can respond to the demands of the reader.
To allow the transmission of data from the tag to the reader, the tag modifies the impedance that it exhibits across the terminals of the antenna circuit. This variation of impedance is detected by the reader on account of the inductive coupling.
A certain number of detection methods are known, in which an operative uses an RFID reader to obtain the location of underground RFID tags.
Detection zone is intended to mean the volume (or by extension the ground surface area) in which an RFID tag is readable by the reader. Communication with the tag is possible if the center of the antenna of the reader is inside the volume and impossible if the center of the antenna is outside this volume. The geometry of the detection zone (shape and dimension) depends on the characteristics of the tag (positioning and sensitivity) and the characteristics of the antenna of the reader, as well as the magnetic field level produced by this antenna.
In patent JP2005181111, RFID tags are fixed on buried pipes. The reader, when it is situated in the detection zone, locates the tag by communicating with the latter. Accordingly, the reader recovers an item of depth information that was stored beforehand in the tag. The reader determines the horizontal positioning of the tag in an approximate manner, based on the fact that the reader is disposed in the detection zone.
Such a method of location turns out to be relatively inaccurate as regards horizontal positioning and does not make it possible to determine the effective depth of the tag. Thus, the item of depth information recovered turns out to be imprecise in the case of a reworking of the surface of the ground or of a displacement of the pipe under the effect of various events, such a variation turning out to be relatively probable for pipes whose lifetime is frequently between 30 and 50 years.
Another known method of location is based on communication between a reader and a low-frequency (between 80 and 120 kHz) RFID tag and the use of inductive type antennas are favored for such applications. The low frequencies correspond to wavelengths that are much greater than the location distances. Such a method is frequently implemented with tags furnished with simple resonators and devoid of electronic chips. The resonator of the tag creates a secondary magnetic field proportional to the primary magnetic field created by the antenna of the reader.
The growth and the decay of the amplitude of the secondary field occur according to a time constant dependent on the quality factor of the resonator. The secondary-field amplitude measured by the reader is relatively low with respect to the amplitude of the primary field. In order to allow measurement of the secondary field, the primary field is emitted only briefly and the measurement of the secondary field is performed during the periods of extinction of the primary field. In order for the amplitude of the secondary field to still be appreciable for a sufficiently long time during the periods of extinction of the primary field, the resonator of the tag exhibits a sufficiently high quality factor (typically between 50 and 100).
The horizontal position or the plumb alignment of the tag is determined by scanning the surface of the ground with the reader. The horizontal position of the tag is identified when the amplitude of the secondary field attains a maximum.
Since tags furnished with a simple resonator are not able to advise the reader as to their configuration, the reader must furthermore solve a problem with two unknowns: the intensity of emission of the secondary magnetic field and the depth of the tag. To determine these two unknowns, the operative places the reader plumb with the tag, and performs two measurements at two predefined heights above the ground.
In order to guarantee the accuracy of measurement of the secondary field, the latter is measured in a repeated manner and an average is calculated over the various measured values. However, on account of the low communication frequency values used, the duration required to carry out the secondary-field measurements turns out in practice to be relatively long for the operator, typically several seconds. Once plumb with the tag, this duration of measurement is doubled in order to measure the secondary field at the two predefined heights. So as not to render location excessively lengthy, the determination of the plumb alignment of the tag must also be carried out by making do with an approximate location of the place for which the secondary field provides a maximum amplitude. Moreover, the accuracy of such a method turns out to be relatively dependent on the external magnetic disturbances in nearby frequencies (for example due to the excitation of other secondary sources), the secondary-field amplitude measured on the reader remaining very small.